Design-for-test methodology for non-scan at-speed testing

While scan-based testing achieves a high fault coverage, it requires long test application times and substantial tester memory, in addition to the overhead in chip area and high test power. Functional testing, on the other hand, suffers from low coverage but can be applied at-speed. In this paper, we propose a novel three-step design-for-test (DFT) methodology which enhances the performance of functional testing to a great extent. In the first step we expand the state space of the circuit beyond functionally reachable space without scan or reset. These new states create conditions to activate/propagate fault effects that are otherwise hard-to-detect. Since structural correlation between D flip-flops (DFFs) of a circuit restricts its state space variation, the second step consists of partitioning the DFFs into different groups that helps to break such correlations. In the third step, we make internal hard-to-observe points in the circuit more observable by directly XORing them with selected primary outputs. This method can be applied at-speed (since no scan shifting is involved) saving significant amount of test application time, with comparable area overhead as scan-based DFT. Our experiments on large ISCAS´89 and ITC´99 benchmarks show that we are able to achieve very high non-scan fault coverages while simultaneously reducing the test application time (114×) as compared to scan based techniques.